Non-mechanical valves allow, through gas injection upstream from an elbow, to circulate the particles in a line. This type of equipment is well known and described in the literature (Knowlton, T. M., “Standpipes and Nonmechanical Valves”, Handbook of Fluidization and Fluid-Particle Systems, Wen-Ching Yang, editor, pp. 571-597. Marcel Dekker, Inc. New York, 2003).
An L-valve is thus described in FIG. 1. The L-valve consists of a vertical line equipped, at the base thereof, with a 90° elbow. If the vertical line is filled with particles, injection of a gas, upstream from this elbow close to the change in direction, allows to promote circulation of the particles in the line. Depending on the pressure conditions imposed at the system terminals, part of the gas injected flows down the line, through the elbow and promotes transport of the particles (FIGS. 2 and 3, configurations A and B). Part of the gas injected can also flow upwards countercurrent to the particle flow (FIG. 3, configuration B). The proportion of upflowing injected gas is adjusted according to the pressure conditions at the valve terminals.
L-valves allow to control the solid circulation when the flow in the vertical line upstream from the gas injection point is not fluidized (i.e. the velocity difference between the gas flow and the particles flow remains below the minimum velocity of fluidization of the particles under the conditions applied). These valves are particularly suitable for group B particles of the Geldart classification, which have a sufficiently high minimum fluidization velocity to allow a high particle flow rate.
Knowlton (Knowlton, T. M., “Standpipes and Nonmechanical Valves”, Handbook of Fluidization and Fluid-Particle Systems, Wen-Ching Yang, editor, pp. 571-597. Marcel Dekker, Inc. New York, 2003) describes a circulating bed system using L-valves (FIG. 4). A circulating fluidized bed (CFB) wherein gas is injected from the bottom (<<Gas In>>) allows the particles to be transported, it carries the gas and the particles to a cyclone C. The particle-free gas leaves the cyclone through a line (<<Gas Out>>) and the separated particles are fed again into the circulating bed through an L-valve. Such a system allows to uncouple the inner solid circulation within the loop from the gas flow in the circulating bed, the pressure available for transport of the particles in the circulating bed varying depending on the amount of aeration gas injected in the L-valve (which allows to vary the pressure recovery in the vertical part of the L-valve).
However, in conventional industrial fluidized bed combustion processes, the technologies used do not allow the inner circulation of solids in the loop to be controlled independently. A circulating bed boiler is shown in FIG. 5 (Nowak et al., IFSA 2008, Industrial Fluidization South Africa, pp. 25-33. Edited by T. Hadley and P. Smit, Johannesburg: South Africa Institute of Mining and Metallurgy, 2008). The combustion air is introduced at the base of the circulating bed and it carries the coal and sand particles to a cyclone. The particles are then recycled to the circulating fluidized bed through a return leg. The return leg is dimensioned so as to promote solid recycling but it does not allow solid circulation control. It is not equipped with L-valves. Sometimes, siphons are positioned on this return leg to prevent gas upflow in the return leg. However, in such systems, the circulation of solids within the loop entirely depends on the amount of combustion air fed into the circulating bed.
There are other means for controlling the circulation of solids. Conditions permitting, mechanical valves can be used on solids. Thus, in the fluidized bed catalytic cracking process (FCC), a method operating at temperatures below 800° C.-850° C., slide valves or plug valves are used to control the circulation between the various enclosures (FIG. 6, Gauthier, IFSA 2008, Industrial Fluidization South Africa, pp. 35-87. Edited by T. Hadley and P. Smit Johannesburg: South Africa Institute of Mining and Metallurgy, 2008).
In FIG. 6, the following elements allowing the FCC process to be implemented are shown:
R1: regenerator No. 1
R2: regenerator No. 2
PV1: mechanical plug valve (plug valve No. 1)
RSV1: mechanical slide valve No. 1 (slide valve No. 1)
RSV2: mechanical slide valve No. 2 (slide valve No. 2)
L: ascending transport line (lift)
FI: feed injection
Q=quench
RR: riser reactor
RS: stripper reactor.
These valves operate on fluidized flows and they have the characteristic feature of controlling the flow by modifying the cross-section of flow, the pressure drop of these valves remaining generally constant and depending only on the conditions of fluidization of the particles upstream from the valve. These valves are particularly well-suited for operation on group A particles of the Geldart classification. Unfortunately, operation of these valves on group B particles is more delicate. In fact, it is impossible to maintain group B particles fluidized without forming large gas bubbles that disturb the flow. Furthermore, the moving parts of these valves exposed to the flow cannot be exposed to very high temperatures (>900° C.).
Chemical looping combustion is a technique allowing to carry out partial or total combustion of gaseous, liquid or solid hydrocarbon feeds, by contact with an active mass such as, for example, a metal oxide at high temperature. The metal oxide then yields part of the oxygen it contains, which takes part in the hydrocarbon combustion. It is therefore no longer necessary to bring the hydrocarbon into contact with air, as in conventional methods. Therefore, the combustion fumes predominantly contain carbon oxides, water and possibly hydrogen, not diluted by the nitrogen in the air. It is thus possible to produce fumes predominantly free of nitrogen and having high CO2 contents (>90 vol. %) allowing to consider CO2 capture and storage. The metal oxide that has taken part in the combustion is then transported to another reaction enclosure where it is contacted with air so as to be oxidized. If the particles from the combustion zone are free of fuel, the gases from this reaction zone are predominantly free of CO2 (which is then present only as traces, for example at concentrations below 1-2 vol. %) and they essentially consist of oxygen-depleted air as a result of the oxidation of the metal particles.
The implementation of a chemical looping combustion process requires large amounts of active masses, metal oxides for example, in contact with the fuel. The metal oxides are generally contained either in ore particles, or in particles resulting from industrial treatments (iron and steel or mining industry residues, used catalysts from the chemical industry or refining). It is also possible to use synthetic materials such as, for example, alumina or silica-alumina supports on which metals that can be oxidized (nickel oxide for example) are deposited. From one metal oxide to the next, the amount of oxygen theoretically available varies considerably and it can reach high values close to 30%. However, depending on the materials, the maximum oxygen capacity really available does generally not exceed more than 20% of the oxygen present. The capacity of these materials to yield oxygen does therefore not exceed globally more than some percents by weight of the particles and it varies considerably from one oxide to the next, generally ranging from 0.1 to 10%, often from 0.3 to 1% by weight. The fluidized bed implementation is therefore particularly advantageous for conducting the combustion. In fact, the finely divided oxide particles circulate more easily in the reduction and oxidation reaction enclosures, and between these enclosures, if these particles are conferred the properties of a fluid (fluidization).
Chemical looping combustion allows to produce energy, in form of vapour or electricity for example. The combustion heat of the feed is similar to that encountered in a conventional combustion. The latter corresponds to the sum of the reduction and oxidation heats in the chemical loop. The distribution among the reduction and oxidation heats greatly depends on the active masses (notably metal oxides) used to achieve chemical looping combustion. In some cases, the exothermicity is distributed between the oxidation and the reduction of the active mass. In other cases, the oxidation is highly exothermic and the reduction is endothermic. In all cases, the sum of the oxidation and reduction heats is equal to the combustion heat of the fuel. The heat is extracted by exchangers arranged inside, on the wall of or as an appendix to the combustion and/or oxidation enclosures, on the fumes lines, or on the metal oxide transfer lines.
The chemical looping combustion principle is now well known (Mohammad M. Hossain, Hugo I. de Lasa, Chemical-looping combustion (CLC) for inherent CO2 separations—a review, Chemical Engineering Science 63 (2008) 4433-4451; Lyngfelt A., Johansson M., and T. Mattisson, “Chemical Looping Combustion, status of development”, in CFB IX, J. Werther, W. Nowak, K.-E. Wirth and E.-U. Hartge (Eds.), Tutech Innovation, Hamburg (2008, FIG. 7)). FIG. 7 diagrammatically shows the “air” reactor (1) where the metal oxides are oxidized, a cyclone (2) allowing the particles to be separated from the gas, and the “fuel” reactor or combustion reactor (3), the seat of the metal oxides reduction. Implementation of a chemical loop in a continuous installation however remains the object of many investigations and developments.
In a conventional circulating bed combustion plant, the inner circulation of solids in the circulation loop depends on the flow of air fed into the circulating bed.
On the other hand, in a chemical looping combustion process, combustion control depends on the amount of solid particles introduced, in contact with the fuel. The circulation of the active mass particles circulating in the combustion enclosure conditions the amount of oxygen available for combustion and the final oxidation state of the active mass at the end of the combustion process. Once combustion complete, the active mass (metal oxide in most cases) has to be oxidized again in contact with air in a distinct enclosure. The circulation between the two enclosures conditions:                the oxygen exchange between the reduction reactor and the oxidation reactor,        the heat exchange between the combustion reactor and the oxidation reactor,        the gas transfer between the two enclosures that has to be minimized.        
It is thus important to be able to control the circulation of solids in the air reactor and in the combustion reactor, independently of the circulating gas flow rates conditioning the transport of particles within each one of these enclosures (air in the oxidation reactor, vapour, hydrocarbons or combustion fumes in the fuel reactor).
The various methods provided so far do not allow independent control of the oxide circulation. Thus, Johannsonn et al. (2006) provide a chemical looping combustion process wherein oxidation of the metal oxides occurs in a circulating bed. The oxides are separated in a cyclone that feeds through discharge the fuel reactor where combustion takes place. The metal oxide is then recycled to the oxidation reactor. Such a system does not allow to control the circulation of metal oxides independently of the air flow rate in the oxidation reactor. The flow rate of metal oxide circulating in the combustion reactor can only be modified by changing the air flow rate in the air reactor.
Another device is described in patent FR-2,850,156. In this case, the two reactors, combustion and oxidation, are circulating beds. Here again, the circulation between the two reactors depends on the flow of gas fed into each enclosure. In both cases, siphons are positioned on the transfer lines allowing transport of the metal oxides. These siphons allow to provide sealing of the gas phases between the two enclosures by preventing gas coming from the oxidation reactor from circulating towards the combustion reactor through the transfer lines, and vice versa. These siphons do not allow to control and to modify the circulation of metal oxides.
Furthermore, the chemical looping combustion method is preferably implemented at high temperature (between 800° C. and 1200° C., typically between 900° C. and 1000° C.). It is therefore not possible to control the circulation of metal oxides using mechanical valves conventionally used in other processes such as FCC.
The object of the invention thus is to provide a novel plant and a novel method allowing to control the circulation of solids in the chemical loop independently of the gas flows circulating in the combustion (reduction) and oxidation enclosures.